Abstract

During the Holocene sea-level rise, the incised Gironde valley has filled with a fluvio-estuarine sediment wedge 55 m thick and 20 km wide at the estuary mouth. This wedge extends landward up to the present tide limit, 150 km from the inlet, where the high tide intersects the upward sloping fluvial profile. The junction between the fluvial profile and the estuarine deposits at base level forms the bayline which is characterized by a facies transition from conglomeratic fluvial point bars to tide-dominated sandmud estuarine point bars.

The valley fill consists of a transgressive tract overlain by highstand regressive estuarine deposits. The transgressive tract comprises three facies associations: fluvial conglomerates at the base, overlain by transgressive estuarine sand and mud, capped by coastal marine sand. The conglomerates form a diachronous sheet-like deposit paving the upward sloping substrate, and are in updip continuity with the present fluvial conglomerates landward of the tide limit. The contact between the fluvial conglomerates and the overlying estuarine sediment is a flooding surface recording the fluvial-tidal bayline transition. The marine sands accumulated in the estuary inlet and indicate a period of maximum flooding of the estuary valley. These are overlain by prograding estuarine muds and tidal bars.

Cored borings have enabled the construction of a stratigraphic model for an estuarine valley fill. When rising base level inundated the incised valley at 10,000 ka, an aggrading fluvio-estuarine sediment wedge developed, which back-stepped landward and onlapped the alluvial plain at the tide limit. Landward of this point, no fluvial aggradation occurred and the fluvial profile remained stationary. Since highstand (5,000 ka), the estuary has been filling with prograding estuarine deposits. As the estuary fills, its volume and cross-sections diminish, bringing about a seaward shift of the tide limit. This will result in a downstream migration of the fluvial conglomerates, as well as fluvial plain aggradation to maintain sufficient slope to ensure downstream continuity of fluvial discharge. This fluvial accumulation will develop the late highstand systems tract, as defined by Posamentier and Vail.